In certain applications such as cryogenic refrigerators used to cool electronic instruments or sensors, it is necessary to drive a piston in a reciprocating fashion. This was initially accomplished with a conventional rotary electric motor in which rotary motion from the motor was converted into linear reciprocating motion through a crank mechanism involving connecting rods and bearings. In order to achieve long life, the motor and bearings must be lubricated, typically through the use of some form of grease. In cryogenic refrigerator applications, the type of greases that are suitable must have very low vapor pressure so that they will not contaminate the working fluid of the refrigerator. To date, all of the known greases provide a certain amount of contamination which limits the life of the refrigerator.
A recent advancement in the art which has been used to extend the life of cryogenic refrigerators is the development of linear motors wherein the refrigerator pistons are moved directly by the motors in linear reciprocating fashion. In linear motors there is no need to convert rotary motion into linear motion. Hence, a simpler device results in which the need for conventional rotary bearings and their lubricating greases is eliminated. Instead, slider bearings made from solid plastic films may be employed, thereby eliminating the need for lubricant greases and resulting in a long-life refrigerator in which the major source of contamination of the working fluid has been removed.
When a linear motor is employed to move a piston in reciprocating fashion, the resultant motion of the center of mass of the moving parts creates a vibrational force on the housing or other structure on which the moving parts are mounted, resulting in undesired mechanical vibration of the entire assembly.
One technique which has been used to reduce vibrations in linear motors is to employ a pair of linear motors disposed on a common axis and operated 180.degree. out of phase with one another. However, not only does such an arrangement result in additional cost due to the second motor and its associated control circuitry, but the size and weight of the overall arrangement are excessively large for some applications.
Another vibration reduction technique which has been employed with linear motors involves a dynamic, or Frahm, counterbalancing scheme. In its classical implementation, a Frahm balancer consists of a mass and a spring that connects the mass to the housing in which vibrations are to be reduced. The spring constant and the mass are selected to result in a natural frequency for the spring-mass subsystem that is exactly equal to the natural frequency of vibration of the linear motor. Under this condition, when the spring-mass balancing subsystem is excited at its natural frequency, it will provide a vibrational force in exact opposition to that provided by the motor on the housing. These vibrational forces cancel one another, and no vibration is transmitted to the housing. An example of an improved form of Frahm counterbalancing system is given in U.S. Pat. No. 4,360,087 to Peter Curwen. Although Frahm counterbalancing systems provide good vibration reduction at or very near the particular frequency for which the system was designed, such systems are not effective over wider ranges of frequencies such as are often encountered in practice.